Backport of [Security] Close cross scripting vulnerability into release/1.19.x (#21346)

* backport of commit c8cb3349fe

* backport of commit 262efd8f15

---------

Co-authored-by: Sarah Alsmiller <sarah.alsmiller@hashicorp.com>

.changelog/21342.txt

@@ -0,0 +1,3 @@
+```release-note:security
+agent: removed reflected cross-site scripting vulnerability
+```

agent/kvs_endpoint.go

@@ -293,7 +293,7 @@ func conflictingFlags(resp http.ResponseWriter, req *http.Request, flags ...stri
 		if _, ok := params[conflict]; ok {
 			if found {
 				resp.WriteHeader(http.StatusBadRequest)
-				fmt.Fprint(resp, "Conflicting flags: "+params.Encode())
+				fmt.Fprintf(resp, "Conflicting flags: %v\n", params.Encode())
 				return true
 			}
 			found = true